ISSN 1402-1757 Synthesis and Characterisation of High ISBN 978 …1314477/FULLTEXT01.pdf · 2019....
Transcript of ISSN 1402-1757 Synthesis and Characterisation of High ISBN 978 …1314477/FULLTEXT01.pdf · 2019....
-
Synthesis and Characterisation of High Entropy Alloy and Coating
Sajid Ali Alvi
Engineering Materials
Department of Engineering Sciences and MathematicsDivision of Materials Science
ISSN 1402-1757ISBN 978-91-7790-394-9 (print)ISBN 978-91-7790-395-6 (pdf)
Luleå University of Technology 2019
LICENTIATE T H E S I S
Sajid Ali A
lvi Synthesis and Characterisation of H
igh Entropy A
lloy and Coating
-
Licentiate thesis
Synthesis and Characterisation of High Entropy Alloy
and Coating
Sajid Ali Alvi
Department of Engineering Sciences and Mathematics
Division of Materials Science
Luleå University of Technolgy
-
Printed by Luleå University of Technology, Graphic Production 2019
ISSN 1402-1757 ISBN 978-91-7790-394-9 (print)ISBN 978-91-7790-395-6 (pdf)
Luleå 2019
www.ltu.se
-
3
Abstract High entropy alloys (HEAs) are a new class of alloys that contains
five or more principal elements in equiatomic or near-equiatomic
proportional ratio. The configuration entropy in the HEAs tends to
stabilize the solid solution formation, such as body-centered-cubic
(BCC), face-centered-cubic (FCC) and/or hexagonal-closed-pack
(HCP) solid solution. The high number of principal elements
present in HEAs results in severe lattice distortion, which in return
gives superior mechanical properties compared to the conventional
alloys. HEAs are considered as a paradigm shift for the next
generation high temperature alloys in extreme environments, such
as aerospace, cutting tools, and bearings applications.
The project is based on the development of refractory high entropy
alloy and film. The first part of the project involves designing high
entropy alloy of CuMoTaWV using spark plasma sintering (SPS)
at 1400 oC. The sintered alloy showed the formation of a composite
of BCC solid solution (HEA) and V rich zones with a
microhardness of 600 HV and 900 HV, respectively. High
temperature ball-on-disc tribological studies were carried out from
room temperature (RT) to 600 oC against Si3N4 counter ball.
Sliding wear characterization of the high entropy alloy composite
showed increasing coefficient of friction (COF) of 0.45-0.67 from
RT to 400 oC and then it decreased to 0.54 at 600 oC. The wear
rates were found to be low at RT (4 × 10 −3 mm3/Nm) and 400 oC
(5 × 10 −3 mm3/Nm) and slightly high at 200 oC (2.3 × 10 −2
mm3/Nm) and 600 oC (4.5 × 10−2 mm3/Nm). The tribology tests
showed adaptive behavior with lower wear rate and COF at 400 oC
and 600 oC, respectively. The adaptive wear behavior at 400 oC
was due to the formation of CuO that protected against wear, and
at 600 oC, the V-rich zones converted to elongated magneli phases
of V2O5 and helped in reducing the friction coefficient.
The second part of the project consists of sintering of novel
CuMoTaWV target material using SPS and depositing
CuMoTaWV refractory high entropy films (RHEF) using DC-
-
4
magnetron sputtering on silicon and 304 stainless steel substrate.
The deposited films showed the formation of nanocrystalline BCC
solid solution. The X-ray diffraction (XRD) studies showed a
strong (110) preferred orientation with a lattice constant and grain
size of 3.18 Å and 18 nm, respectively. The lattice parameter were
found to be in good agreement with the one from the DFT
optimized SQS (3.16 Å). The nanoindentation hardness
measurement at 3 mN load revealed an average hardness of 19 ±
2.3 GPa and an average Young’s modulus of 259.3 ± 19.2 GPa.
The Rutherford backscattered (RBS) measurement showed a
gradient composition in the cross-section of the film with W, Ta
and Mo rich at the surface, while V and Cu were found to be rich
at the substrate-film interface. AFM measurements showed an
average surface roughness (Sa) of 3 nm. Nano-pillars of 440 nm
diameter from CuMoTaWV RHEFs were prepared by ion-milling
in a focused-ion-beam (FIB) instrument, followed by its
compression. The compressional yield strength and Young’s
modulus was calculated to be 10.7 ± 0.8 GPa and 196 ± 10 GPa,
respectively. Room temperature ball-on-disc tribological test on the
CuMoTaWV RHEF, after annealing at 300 oC, against E52100
alloy steel (Grade 25, 700-880 HV) showed a steady state COF of
0.25 and a low average wear rate of 6.4 x 10-6 mm3/Nm.
-
5
Acknowledgements
This licentiate thesis is part of the project “thermo-mechanical and
tribology infrastructure (TMTEST)” which is focused on the
development of new metal/ceramic composites for high temperature
applications and funded by Swedish Foundation of Strategic Research
(SSF). I am extremely thankful for being part of this project.
I would like to extend by deepest gratitude to my supervisor Farid Akhtar
for giving me the opportunity to work in this project and for being a
source of motivation towards innovative research work. I would also like
to thank my co-supervisor Marta-Lena Antti for her fruitful feedback and
supervision during the last two years.
I would like to thank Lars Frisk and Johnny Grahn for their tireless effort
in helping out in different labs, and for always bringing laughter and
positive work atmosphere in the department.
To all my colleagues at the Division of Materials Science, I am extremely
grateful for a very good and positive working environment. I would
specially like to thank Alberto Vomiero for collaborating with me on thin
film synthesis and giving valuable feedback to my work. I would like to
thank Mojtaba Gilzad for great talks on life and research, and helping me
out with developing new coatings. Many thanks to Magnus Neiktar,
Daniel Hedman, Kamran Saeidi, Pedram Ghamgosar and Federica
Rigoni for their collaboration in different research work and feedback. I
am also grateful for fruitful discussion and help from Hanzhu Zhang,
Kritika Narang, Zhejian Cao and Viktor Sandell. I am thankful to my
master students Oriol Artola and Anton Ek for helping me out with
experimental work on different projects.
Lastly, I would like to thank my parents and brother for their continuous
support throughout my life and their unconditional love.
-
6
-
7
List of Papers
For this thesis, the following papers have been included:
I. Sajid Alvi, Farid Akhtar “High temperature tribology of
CuMoTaWV high entropy alloy” Wear. 426 (2019) 412-419.
II. Sajid Alvi, Dariusz Jarzabek, Mojtaba Gilzad Kohan, Daniel
Hedman, Marta Maria Natile, Alberto Vomiero, Farid
Akhtar “Synthesis and mechanical characterization of
CuMoTaWV high entropy film by magnetron sputtering”
(Submitted to ACS Applied Materials & Interfaces).
List of papers not included in this thesis:
I. Sajid Ali Alvi, Farid Akhtar “High temperature tribology of
polymer derived ceramic composite coatings” Scientific
reports. 8 (2018) 15105.
II. Kamran Saeidi, Sajid Ali Alvi, Frantisek Lofaj, Valeri
Ivanov Petkov, Farid Akhtar “Advanced Mechanical
Strength in Post Heat Treated SLM 2507 at Room and High
Temperature Promoted by Hard/Ductile Sigma
Precipitates” Metals. 9 (2019) 199.
III. Sajid Ali Alvi, Pedram Ghamgosar, Federica Rigoni,
Alberto Vomiero, and Farid Akhtar “Adaptive
nanolaminate coating by atomic layer deposition”
(Submitted to Solid Thin Films).
IV. Sajid Ali Alvi, Kamran Saeidi, Farid Akhtar ”High
temperature tribology of selective laser melted 316L
stainless steel” (Submitted to Wear).
-
8
-
9
Table of Contents Abstract .................................................................................................. 3
Acknowledgements ................................................................................ 5
1. Introduction .................................................................................. 11
2. Introduction to High entropy alloys ............................................. 12
2.1. The core effects .................................................................... 13
2.1.1. High-entropy effect ...................................................... 13
2.1.2. Lattice distortion effect ................................................. 14
2.1.3. Sluggish diffusion ......................................................... 15
2.1.4. Cocktail effect .............................................................. 16
2.2. Alloy preparation .................................................................. 17
2.2.1. Arc Melting (Liquid state) ............................................ 17
2.2.2. Spark plasma sintering (Solid state) ............................. 18
2.2.3. Magnetron sputtering (Gaseous phase) ........................ 20
2.3. Refractory high entropy alloys ............................................. 21
2.4. Refractory high entropy films .............................................. 25
2.5. Existing research gaps .......................................................... 28
3. Experimental methods .................................................................. 29
3.1. Spark plasma sintering (SPS) ............................................... 29
3.2. Magnetron sputtering ............................................................ 30
3.3. Microhardness measurement ................................................ 32
3.4. Nanoindentation ................................................................... 32
3.5. Nano-pillar compression tests .............................................. 32
3.6. Tribological studies .............................................................. 33
3.7. Scanning electron microscopy (SEM) .................................. 34
3.8. XPS measurements ............................................................... 34
3.9. Rutherford backscattering spectrometry ............................... 35
-
10
3.10. Atomic force microscopy (AFM) .................................... 35
3.11. DFT calculations .............................................................. 35
4. Summary of appended papers ...................................................... 36
Paper I .............................................................................................. 36
Paper II ............................................................................................. 37
5. Conclusions .................................................................................. 39
6. Future work .................................................................................. 41
7. References .................................................................................... 43
-
11
1. Introduction
1.1. Aims of this project
The forefront of research in materials science has been developing
new materials with good strength and stiffness at higher
temperatures for applications, such as aerospace and jet engine.
Recently, development of high entropy alloys has made it possible
to achieve superior mechanical properties at high temperatures
compared to conventional alloys. However, less attention has been
given to the tribological properties of high entropy alloys and
films. The aim of this licentiate work is to develop refractory high
entropy alloy (RHEA) using spark plasma sintering (SPS) and
understand the effect of microstructural characteristics on the high
temperature tribological properties. The second aim is to develop
refractory high entropy films (RHEF) using magnetron sputtering
with a single partially sintered target and study the effect of a single
target on nanocrystallinity and mechanical properties. Keeping
these aims in mind, we have developed a CuMoTaWV RHEA
using SPS and studied its high temperature tribological properties
up to 600 oC. Secondly, a single partially sintered target by SPS
has been used to develop CuMoTaWV RHEF and its mechanical
properties, such as hardness, nano-pillar compression and room
temperature tribological studies have been investigated.
1.2. Research questions
The following research questions have been addressed in this
thesis:
What are the microstructural characteristics of RHEA after
SPS and its affect on the tribological properties? Secondly,
how does the selection of different elements effects the
properties at high temperature?
How does the use of single partially sintered target effects
the nanocrystallinity and mechanical properties, such as
hardness, tribology and nano-pillar compression in RHEF?
-
12
2. Introduction to High entropy alloys In the past, alloy design has consisted of combining single
principal element with minor alloying elements to enhance the
mechanical, chemical and/or structural properties. The first work
on developing an alloy with multi principle elements was first
reported in 2003 by Yeh et al. [1], Cantor et al. [2], and
Ranganathan et al. [3] and showed superior properties to
conventional alloys, leading to wide spread popularity of
multicomponent alloys as high entropy alloys (HEAs) among
researchers around the world.
HEAs are defined as alloy system consisting of five or more
principal elements with a concentration of each principal element
between 35 and 5 at.%. The mixing of multiple components results
in high entropy of mixing, expressed by [4,5]:
∆𝑆𝑚𝑖𝑥 = −𝑅 ∑ 𝑐𝑖 ln 𝑐𝑖𝑛𝑖=1 (i)
Where, R is the gas constant, ci the molar fraction of ith element,
and n the total number of constituent elements. The configurational
entropy has the highest contribution to the entropy of mixing of
high entropy alloys as compare to other sources, such as
vibrational, electronic and magnetic entropy [6]. For an equiatomic
HEA, the entropy would reach a maximum with the
configurational entropy (∆𝑆𝑐𝑜𝑛𝑓) expressed by:
∆𝑆𝑐𝑜𝑛𝑓 = 𝑅 ln 𝑛 (ii)
where n is the no of elements. As a result, the ∆𝑆𝑐𝑜𝑛𝑓 for an
equimolar alloy with 3, 5, 6, and 9 elements will result in to 1.10R,
1.61R, 1.79R and 2.20R, respectively. Thus, in order for a
multicomponent alloy to be classified as HEA, the lower limit of
∆𝑆𝑐𝑜𝑛𝑓 has been suggested to be 1.5R.
The increase in entropy of mixing in multicomponent alloy results
in decreasing the Gibbs free energy of mixing (∆𝐺𝑚𝑖𝑥) leading to formation of stable solid solution of face-centered cubic (FCC),
-
13
body-centered cubic (BCC) or hexagonal close pack (HCP) crystal
structures. The relation between ∆𝑆𝑚𝑖𝑥 and ∆𝐺𝑚𝑖𝑥 can be given
by the equation (iii) [7]:
∆𝐺𝑚𝑖𝑥 = ∆𝐻𝑚𝑖𝑥 − 𝑇∆𝑆𝑚𝑖𝑥 (iii)
Where ∆𝐻𝑚𝑖𝑥 is enthalpy of mixing and T is the absolute temperature. The phase formation is determined by the
competition between ∆𝐻𝑚𝑖𝑥 and 𝑇∆𝑆𝑚𝑖𝑥. With increasing
temperature, a higher 𝑇∆𝑆𝑚𝑖𝑥 term becomes more dominant and
will lead to formation of solid solution.
2.1. The core effects The composition of HEAs from five or more principal elements
are complex as compare to conventional alloys. Yeh [8] suggested
four core effects that describe the properties of HEAs: (1) high
entropy effects, (2) sluggish diffusion, (3) severe lattice distortion,
and (4) cocktail effects.
2.1.1. High-entropy effect
The high-entropy effect was first proposed by Yeh, which
described the stabilization of high-entropy phases with the
formation of solid solution phases. According to Gibbs phase rule,
the number of phases (P) in a given alloy at constant pressure in
equilibrium condition can be expressed as:
P = C + 1 – F (iv)
where, C is the number of component and F is the maximum
number of thermodynamic degrees of freedom in the system. For
example, in a 6-component system at a given pressure, it is
expected to have 7 phases at any given reaction. However,
compositions satisfying the HEAs-formation criteria will form
solid-solution phases [2,9,10].
According to the classical physical-metallurgy theory, a solid
solution phase is based on one element (solvent) and contains other
minor elements (solutes). However, in HEAs, it is difficult to
-
14
differentiate between solvent and solute because of their equimolar
compositions. According to many researchers, HEAs can only
form simple FCC or BCC solid solution, and the number of phases
formed are much smaller than what Gibbs phase rule in equation
(iv) allows [8,11]. Thus, indicating that high entropy of alloys
expands the solubility limit between chemically compatible
elements. This phenomenon can be explained with reference to
equation (iii), where the mixing entropy competes with the mixing
enthalpy. Furthermore, the 𝑇∆𝑆𝑚𝑖𝑥 term becomes more dominant at high temperatures, resulting in minimizing the free energy and
forming simple multi-element solid-solution phases.
2.1.2. Lattice distortion effect
In HEAs, each element has the same possibility of
occupying a lattice site, and since the size of each element can be
different in many cases, this can lead to formation of severed lattice
distortion. Furthermore, such lattices with many principal elements
are highly distorted, as all the atoms are solute atoms with different
atomic sizes, as shown in Figure 2.1. Such severe lattice distortion
can lead to higher strength and hardness of HEAs [12]. Yeh et al.
[13] also proved the lattice distortion in HEAs by studying its
effect on decreasing trend in X-ray diffraction (XRD) intensities
of CuNiAlCoCrFeSi alloy system with systematic addition of
principal elements.
-
15
Figure 2.1: Schematic illustration of BCC crystal structure: (a) perfect
lattice of Cr; (b) distorted lattice with the addition of V (for example:
Cr-V solid solution); (c) extremely distorted lattice caused of atoms
with different atomic sizes randomly distributed in the crystal lattice
with same probability to occupy the lattice position in HEA system
(AlCoCrFeNiTi0.5[14]) [10].
2.1.3. Sluggish diffusion
The equilibrium partitioning among the phases is attained
through phase transformations that depend on atomic diffusion.
However, according to Yeh [8], diffusion in HEAs are hindered
due to lattice distortion, thus limiting the diffusion rate. During the
conventional casting of HEAs, the phase separation during cooling
is inhibited at a higher temperature and the resulting alloy has
nano-precipitates in the matrix. Such formation of nano-
precipitates in HEAs results in higher recrystallization temperature
and activation energies during deformation. The sluggish diffusion
-
16
in coatings, such as by magnetron sputtering, results in the
formation of nanocrystalline or amorphous phase for a higher
number of principal element as the growth and nucleation of
crystalline phases are inhibited, as shown in Figure 2.2 [15]. Such
structural variation with a different number of principal elements
can be easily exploited in HEAs to promote the mechanical,
physical and chemical properties.
Figure 2.2: XRD analysis showing structural evolution of two to
seven elements sputtered films [15].
2.1.4. Cocktail effect
According to Yeh [8], the incorporation of multi-principle
elements in HEAs can be considered as an atomic-scale composite.
This composite effect is characterized by the basic features and
interactions between all the elements present in HEA along with
the indirect effects of different elements on the microstructure.
Thus, the term cocktail effect, first mentioned by Ranganathan [3],
comes from the properties resulting from combination of principal
elements that cannot be obtained from only a single principle
element. For example, light elements are selected for developing a
HEA with lighter density, such as Al, Cr Cr, Cu, Fe, Ni and Si. For
oxidation resistance at higher temperatures, Si, Al and Cr are used
[16]. To increase the strength, Al is added to a multi-element
-
17
system of Co, Cr, Cu, Fe and Ni, which promotes the formation of
BCC along with inducing higher strength, as shown in Figure 2.3
[17,18].
Figure 2.3: Effect of aluminum addition on the cast hardness of
AlxCuCoNiCrFe alloys. A, B and C refers to that hardness, FCC lattice
constant and BCC lattice constant, respectively [8].
2.2. Alloy preparation 2.2.1. Arc Melting (Liquid state)
The most widely used processing technique for HEAs
development is arc melting technique. The typical schematic of arc
melting method is shown in Figure 2.4. The electrode temperature
of arc melting furnace can go in excess of 3000 oC, which can be
controlled by adjusting the electrical power. Thus, arc melting
technique is beneficial towards homogenous melt mixing of
elements with high melting points, such as refractory elements.
However, heterogeneous microstructure (dendritic with
interdendritic segregation) in the resulting HEAs are induced by
the thermal gradient from surface to center during solidification,
which makes the microstructure and properties of as-cast HEAs
less controllable. Furthermore, large differences in elemental
-
18
melting temperatures can result in elemental segregation in
refractory HEAs casted from liquid state [19,20]. The use of arc
melting technique restricts the as-casted alloy shapes to rods and
buttons, thus complex shapes are difficult to cast for advanced
applications.
Figure 2.4: A schematic representation of the arc melting process [21].
2.2.2. Spark plasma sintering (Solid state)
Mechanical alloying (MA) is a process consisting of high-
energy ball milling of elemental powders at room temperature for
a long duration of time resulting in diffusion of species into each
other to obtain homogenous alloy. Murty’s research group was the
first to demonstrate the use of MA to synthesize nanostructured
equiatomic AlFeToCrZnCu HEA, where each nanoparticle
obtained was equiatomic in its composition [22].
The HEAs developed by MA are in powder form and needs
sintering in order to get a dense component. In order to avoid grain
growth of nanocrystalline alloy powder during conventional
sintering process, spark plasma sintering (SPS), also known as
pulsed current processing (PCP), is used to obtain nanocrystalline
-
19
alloys. SPS employs the usage of high amperage pulse current of
up to 5000 A with simultaneously applying uniaxial pressure of up
to 100 MPa through powder mixture packed in a graphite die, as
shown in Figure 2.5. Graphite paper is usually added between the
sample and the graphite die to avoid unnecessary reaction of the
powder mixture with the die. The combination of high current
pulse, uniaxial pressure and vacuum/inert atmosphere leads to
consolidation of powders at very short time.
Figure 2.5: Schematic of spark plasma sintering [23].
Compared to conventional liquid state alloy development of HEA,
such as arc melting, SPS has been used to develop nanocrystalline
HEAs [24–26]. Recently, researchers have employed SPS to
develop composite HEAs for tribological applications. Zhang et al.
[27] developed CoCrFeNi-Ag-BaF2/CaF2 composite HEA and
showed low frictional (COF: 0.2 to 0.25) and wear (wear rate: 2 x
10-5 mm3/Nm to 5 x 10-5 mm3/Nm) behavior against Inconel 718
counter balls from RT to 800 oC. Similarly, Cao et al. [28]
developed NiCoCrAl-Ag-MoS2-LaF3-CeF3 composite HEA using
SPS and studied the high temperature tribology from RT to 800 oC
against Si3N4 balls, and showed adaptive frictional (COF: 0.3-0.4)
and wear behavior (wear rate: 2.05 x 10-5 mm3/Nm to 5.18 x 10-6
mm3/Nm).
-
20
2.2.3. Magnetron sputtering (Gaseous phase)
Magnetron sputtering is a type of physical vapor deposition
(PVD) technique that is used for deposition of thin films on a
substrate through sputtering away atoms from target material
(elemental or alloy) under the bombardment of charge Ar+ ions.
Simple schematics of sputtering process is shown in Figure 2.6.
There are two types of sputtering techniques commonly used:
direct current (DC) sputtering and radio frequency (RF) sputtering.
DC sputtering is a technique where DC bias is applied between the
target and substrate to deposit the atoms generated from the target
material. Controlling power, bias voltage, and the argon pressure
controls the deposition rate of film onto the substrate. The second
type of sputtering (RF sputtering) is used for deposition of
insulating materials, where plasma can be maintained even at
lower argon pressure resulting in fewer gas collisions and line of
sight deposition [29].
Figure 2.6: Schematic of magnetron sputtering deposition process [30].
Many researchers have explored magnetron sputtering technique
in the last decade to develop thin films of high entropy alloy, high
entropy nitride and high entropy carbides [31]. The major
advantage of using magnetron sputtering for developing high
-
21
entropy film is the control of reactive gases (such as N2, O2 or
CH4/C2H2) [32–36], nanocrystallinity [37] and influence of
deposition condition (such as substrate temperature and substrate
bias) [38–40]. Such control on different parameters results is
obtaining high entropy films with superior properties, such as high
hardness [32,41–43], high compressive strength [44–46],
corrosion resistance [39,47], wear resistance [36,48–51], unique
physical characteristics [52–54] and high temperature stability
[48,55,56].
2.3. Refractory high entropy alloys Refractory metals consisting of W, Mo, Ta and Nb, and its
alloys are critical for applications that require high temperature
strength due to their high temperature melting point (Tm). The
definition of refractory metals also extends to include Ti, Zr, Hf,
V, Re, Ru, Os and Ir. Some of the key properties, such as melting
point, density and crystal structure, of refractory metals are listed
in Table 2.1. Among the refractory metals, Ti is the lightest
element with a density of 4.507 g/cm3 while Ir is the heaviest with
a density of 22.65 g/cm3.
The initial research on HEAs was focused on 3-d transition metal
(TM) of Co, Cr, Cu, Fe, Mn, Ni, Ti, and V [57]. However, to
develop HEA for high temperature applications, TM HEAs fail to
operate above 600 oC [58]. Following the founding motivation,
Senkov et al. in 2010 first reported the development of refractory
high entropy alloys (RHEAs) based on five refractory elements of
Mo, Nb, Ta, V, and W that showed superior high temperature
(above 1000 oC) mechanical properties [19,59]. Subsequently,
different researchers incorporated other refractory elements from
Group IV (Ti, Zr, and Hf), Group V (V, Nb, and Ta), and group VI
(Cr, Mo, and W) to further increase the mechanical properties of
RHEAs at wide temperature ranges [58]. RHEA tend to form a
BCC crystal structure and may also contain Laves phases if Cr or
V are also present in the alloy. The densities of RHEA can range
from 5.6 to 13.7 g/cm3, which is higher than TM HEAs with
-
22
densities ranging from 5.0 to 9.0 g/cm3. Thus, small amounts of
non-refractory elements, such as Al, Si, Co or Ni, are added to
lower the density or increase the ductility of RHEA at room
temperature [60].
Table 2.1: Melting point, density and crystal structure of
different refractory elements [61].
Element Melting point
(oC) ρ (g/cm3)
Crystal
Structure
Titanium (Ti) 1668 4.507 Polymorphic
(HCP/BCC)
Zirconium (Zr) 1855 6.511 Polymorphic
(HCP/BCC)
Hafnium (Hf) 2233 13.31 Polymorphic
(HCP/BCC)
Vanadium (V) 1910 6.11 BCC
Niobium (Nb) 2477 8.57 BCC
Tantalum (Ta) 3017 16.65 BCC
Chromium (Cr) 1907 7.14 BCC
Molybdenum
(Mo) 2623 10.28
BCC
Tungsten (W) 3422 19.25 BCC
Rhenium (Re) 3186 21.06 HCP
Ruthenium (Ru) 2334 12.37 HCP
Osmium (Os) 3033 22.61 HCP
Rhodium (Rh) 1964 12.45 FCC
Iridium (Ir) 2466 22.65 FCC
RHEAs are mostly developed using arc melting technique.
However, arc melting can lead to typical solidification defects,
such as elemental segregation, micro-/macro-segregation, pores
and residual stresses due to large differences in elemental melting
-
23
temperatures [19,20]. Recently, SPS has been employed to develop
dense and fine-grained RHEA from elemental or pre-alloyed
powders [26][62].
2.3.1. Properties
2.3.1.1.Mechanical
RHEAs are considered as future alloys for high
temperature application to replace the current Ni-based super-
alloys due to their high mechanical properties at different
temperature ranges, as shown in Figure 2.7 [63]. Therefore, there
is a great research effort to identify alloy compositions that have
good overall strength and ductility from RT to high temperatures
(above 1000 oC) [58]. The first reported RHEA of MoNbTaW and
MoNbTaWV showed high compressive yield strength of 421-506
MPa and 656-735 MPa, respectively, in the temperature range of
1400-1600 oC [59]. Similarly, most of the RHEA reported have
been found to have good mechanical properties at temperatures
higher than 600-800 oC. However, it was found to be brittle at RT
due to interstitial elements segregating to grain boundaries during
solidification or annealing. Addition of Group IV elements, such
as Hf, Ti and Zr, generally enhances the ductility, and addition of
Group V and VI elements and/or Al increases the strength of the
materials but promotes low ductility. Consequently, new RHEAs,
such as addition of Ti in MoNbTaW and MoNbTaWV, were
designed to increase the RT ductility while maintaining high
compressive strength at a broad range of temperatures [64,65].
Recently, researchers have developed RHEAs with the addition of
carbon and Si to prepare RHEA with the formation of MC carbides
(M: Hf, Nb, Ti, or Zr) and M5Si3 phases, respectively [66–69]. The
formation of carbides was found to increase the ductility and work
hardening at RT for low carbon content, such as
C0.1Hf0.5Mo0.5NbTiZr [67]. In another work by Guo et al., the
effect of adding small amounts of Si (~2.5 at.%) in
Hf0.5Mo0.5NbTiZr to form M5Si3, which resulted in grain
refinement and improved compressional strength and ductility
-
24
[66]. Another strategy for RHEA seeks to develop light-weight
RHEA by reducing the density while keeping superior mechanical
properties using Cr, Nb, Mo, Ti, V, and Zr as principal elements,
such as AlxTiVZry alloys, reducing the densities to 5.6-5.7 g/cm3
[70,71]. Similarly, Senkov et al. developed a light-weight RHEA
of CrNbTiVZr with a density of 6.57 g/cm3 and had a yield
strength of 187 MPa and 94 MPa at 600 oC and 800 oC,
respectively [60,72].
Recently, RHEAs of HfNb0.18Ta0.18Ti1.27Zr and Ta0.4HfZrTi have
been developed to improve the work hardening and tensile
ductility through deformation-induced phase transformations, also
known as transformation induced plasticity (TRIP) [73,74]. This
strategy resulted in yield stress of 400-500 MPa through forming
strain-localized regions of HCP phase in Ta0.4HfZrTi and α’’
martensite in HfNb0.18Ta0.18Ti1.27Zr.
Figure 2.7: Comparison of yield stress variation at different
temperatures of different RHEAs with two Ni-base superalloys
Inconel 718 and Mar-M247 [58]. RT: room temperature; SPS: spark
plasma sintering; TRIP: transformation-induced plasticity.
-
25
2.3.1.2.Tribological behavior
The tribological studies of RHEA are scarce. Poulia et al. [75]
studied the dry sliding wear behavior of MoTaWNbV RHEA
(7576 HV0.5) developed by arc-melting against alumina and steel
counter-balls at RT and compared its wear behavior with Inconel
718. The wear rate of MoTaWNbV RHEA was found to be 1.57
x 10-4 mm3/Nm and 2.3 x 10-4 mm3/Nm against alumina and steel
counter-ball, respectively. In contrast, Inconel 718 had a higher
wear rate of 4.57 x 10-4 mm3/Nm and 8.3 x 10-4 mm3/Nm against
alumina and steel counter-ball, respectively. In subsequent work,
Mathiou et al. [76] studied the dry sliding wear of MoTaNbZrTi
against alumina and steel counter-ball at RT with different sliding
distances and compared it to MoTaNbWV RHEA. The wear rate
of MoTaNbZrTi with sliding distance from 400 m to 2000 m was
reported to have decreasing trend from 0.199 x 10-4 mm3/Nm to
0.141 x 10-4 mm3/Nm against steel counter-ball, and an increasing
trend from 1.13 x 10-4 mm3/Nm to 1.41 x 10-4 mm3/Nm against
alumina counter-ball. In all tribological test conditions, the wear
rate of MoTaNbZrTi RHEA was found to be lower than
MoTaNbWV RHEA. However, there is still a research gap is
studying the high temperature tribological behavior of RHEA.
2.4. Refractory high entropy films In the last 5 years, there have been few research work
reported on refractory high entropy films (RHEFs) by magnetron
sputtering. Although, more work has been done on nitrides of
RHEFs [51,77,78]. Majority of the research on RHEFs has been
reported on NbMoTaW composition [37,44,45,79,80], along with
few studies on HfNbTiVZr [38] and TiTaHfNbZr [81]
compositions.
2.4.1. Properties
2.4.1.1.Mechanical
RHEFs developed using magnetron sputtering leads to the
formation of nanocrystalline grains, which in return enhances the
-
26
mechanical properties, such as hardness and nano-pillar
compressional strength. Zou et al. [44,45] studied the
nanocrystalline (nc) pillar compression of NbMoTaW RHEF from
RT to 600 oC, and reported high compressional strength up to 10
GPa (RT) and 5 GPa (600 oC). It was reported that with the
decrease in nc-pillar diameter, the highest compressional strength
of 10 GPa was obtained at ~70-100 nm, and were found to be
higher than all the reported nc-metals in the literature [82].
Furthermore, even after annealing of NbMoTaW RHEF at 1,100 oC for 3 days, the film retained its needle-like morphology and nc-
pillar compressional strength. The Ashby inspired map of specific
strength versus test temperature of pillar compression of different
metals and alloys are shown in Figure 2.8 [45]. Furthermore, even
after annealing of NbMoTaW RHEF at 1,100 oC for 3 days, the
film retained its needle-like morphology and nc-pillar
compressional strength.
Figure 2.8: Ashby inspired specific strength versus temperature plot of
pillar compression of different nc-metals, showing that nc-HEA exhibit
the highest strength-to-density ratio at different temperatures [45].
Feng et al. [79] studied the effect of film thickness on mechanical
properties of NbMoTaW RHEF using nanoindentation technique.
It was reported that with the increasing film thickness (100 nm to
-
27
2000 nm), nanoindentation hardness and elastic modulus
decreased from ~16 GPa and ~200 GPa to ~11 GPa and ~185 GPa,
receptively. This decrease in mechanical properties was attributed
to the grain coarsening as the grain size increased from 10 nm to
~32 nm with increasing film thickness. Similarly, Kim et al. [37]
studied the effect of using single alloyed target sputtering versus
the use of elemental co-sputtering of the target on the
nanocrystallinity and mechanical properties of NbMoTaW RHEF.
A partially sintered alloy target of NbMoTaW was developed
using hot pressing followed by sintering at 1400 oC. The mean
grain size of deposited NbMoTaW RHEF was found to be 15.8
nm, whereas Zou et al. [44] reported a grain size of 150 nm of same
composition using elemental co-sputtering from multiple targets.
Fritze et al. [38] studied the influence of deposition/substrate
temperature on the phase evolution and mechanical properties of
HfNbTiVZr RHEF. With the increase in substrate temperature
from RT to 450 oC, a transition from amorphous phase to BCC
matrix with C14 or C15 Laves phase precipitates were observed.
Furthermore, the nanoindentation hardness also increased from 6.5
GPa to 9.2 GPa with increasing substrate temperature.
2.4.1.2.Tribological behavior
Tuten et al. [81] reported on tribological studies of
TiTaHfNbZr RHEF deposited on Ti-6Al-4V alloy substrate using
RF magnetron sputtering technique for biomedical applications.
The resulting RHEF showed an average hardness and Young’s
modulus of 12.51 GPa and 181 GPa, respectively. The sliding test
against alumina counter-ball showed low frictional coefficient
from 0.1 to 0.2 with increasing normal load from 1 to 3N. The low
frictional coefficient has been attributed to the nanocrystallinity of
RHEF. There is still a research gap for tribological studies of
RHEF, which needs to be fulfilled in the coming future, especially
at higher temperatures.
-
28
2.5. Existing research gaps Although the research on refractory high entropy alloys
(RHEA) and refractory high entropy films (RHEFs) is rather new,
many researchers have reported its mechanical
(hardness/tensile/compression/creep) performance at wide
temperature ranges as alternate to Ni-based super alloys. However,
the work on high temperature tribological studies of RHEAs and
RHEFs has received less attention. The underlying mechanism of
wear at different temperatures also needs further investigation in
relation to microstructure and crystallographic orientation of
RHEAs and RHEAFs. Furthermore, the selection of different
principle element’s effect on the tribological properties of RHEA
and RHEF at different temperature ranges needs to be investigated.
Thus, keeping this research gap in mind, this thesis tries to cover
the high temperature wear mechanism in CuMoTaWV RHEA and
RT tribological behavior in CuMoTaWV RHEF.
-
29
3. Experimental methods
3.1. Spark plasma sintering (SPS)
Figure 3.1: Schematic of alloy development from powder mixture to
sintering using SPS.
Equimolar mixture of Cu, Mo, Ta, W and V powders were
fed into a plastic vial and mixed for one hour using ball milling.
Alumina balls were used for ball milling process with a ball to
powder ratio of 3:1. Details of the starting powders are shown in
Table 3.1. Nano powder of W was used in paper I due to its low
diffusion coefficient. The resulting powder mixture was fed into
graphite die with a diameter of 12 mm and 76 mm for paper I and
paper II, respectively. The SPS in paper I was carried out in
ultrahigh vacuum environment using SPS 825 (Dr. Sinter Spark
Plasma Sintering System), and SPS in paper II was used under
argon atmosphere using SPS 530ET (Dr. Sinter Spark Plasma
Sintering System) to consolidate powder mixture for target
development. A heating rate of 50 oC/min and holding time of 10
mins was used in paper I, while a heating rate of 100 oC/min was
used for target development in paper II. The details of heating rate
-
30
and holding time parameters for both papers are shown in Figure
3.2.
Table 3.1: Particle size distribution of starting material in
paper I and paper II
Cu Mo Ta W V
Paper I 10 µm 3-7 µm 44 µm 70 nm 44 µm
Paper II 10 µm 3-7 µm 44 µm 44 µm 44 µm
Figure 3.2: Schematic of heating cycles for SPS in paper I and paper II.
3.2. Magnetron sputtering An equimolar CuMoTaWV target for sputtering deposition
with a diameter of 76 mm and a thickness of 3 mm was developed
using SPS 530ET (Dr. Sinter Spark Plasma Sintering System), as
shown in Figure 3.3. Direct current (DC) magnetron sputtering
(Moorfield, UK) was used to deposit CuMoTaWV RHEF on
silicon and 304 stainless steel substrate in paper II.
-
31
Figure 3.3: Target prepared at 1000 oC using SPS.
The RHEF was deposited on silicon substrate for cross-section
analysis, nanoindentation hardness measurement, nano-pillar
compression test, Rutherford backscattering analysis (RBS) and
X-ray photoelectron spectra (XPS) analysis; and on 304 stainless
steel substrate for XRD analysis and tribological tests. Sheet metal
substrate of 304 stainless steel with a thickness of 5 mm was cut
into 20 x 20 mm and diamond polished up to 0.4 µm, followed by
cleaning with ethanol in ultrasonicator for 30 mins and dried in an
oven at 80 oC for 30 mins. Silicon substrate with a thickness of 2
mm and dimensions of 5 x 5 mm were used after rinsing with
ethanol and distilled water followed by drying in an oven at 70 oC.
The parameters for film deposition, as shown in Table 3.2, were
used to get a film thickness of ~900 nm. A substrate temperature
of 500 oC was used to enhance the sputtering yield.
-
32
Table 3.2: Sputtering parameters for deposition of CuMoTaWV high
entropy film
Substrate temperature (oC) 500
Atmosphere Ar
Gas Flow (sccm) 20
Substrate rotation speed (rpm) 7
Deposition pressure (mPa) 1.1610-3
Deposition power (W) 150
Deposition duration (min) 120
Deposition rate (nm/min) 7.5
3.3. Microhardness measurement In paper I, microhardness measurements were performed
using Vickers microhardness (Matsuzawa, MXT-CX) equipped
with diamond indenter at 200 g load. The samples were diamond
polished up to 0.4 µm to get a smooth surface for hardness
measurement. At least 8 indentations were performed at different
location of interest.
3.4. Nanoindentation In paper II, hardness measurements were performed using
nanoindenter (Micro Materials, UK) at 10 mN load. The
nanoindentation tests were performed at loading, unloading and
duel time of 25 sec, 20 sec and 10 sec, respectively.
Nanoindentation measurements were performed to obtain force-
displacement curve at 12 different location and the average
hardness and Young’s modulus were reported in the paper.
3.5. Nano-pillar compression tests In paper II, nano-pillars were developed in CuMoTaWV
RHEF using focused-ion beam (FIB) for compression tests. For
each pillars, firstly, a ring was etched using high current (1 nA)
with the outer and inner diameter set to 30 µm and 8 µm,
respectively. Lastly, fine etching of a pillar with current lower than
320 pA was used to mill 440 nm diameter pillars. The pillars were
then compressed using Anton Paar ultra-nanoindentation tester in
-
33
the load control mode with a maximum force set to 3 mN using a
flat punch with a diameter of 20 µm. The loading and unloading
rates were set to 1 mN/min and 3 mN/min, respectively. The data
acquisition rate was set to 50 Hz. The value of the compression
strength was determined as the engineering strain at which 0.2%
plastic deformation occurs. Young’s modulus was determined
from the linear fit to the data for which engineering stress was
lower than 0.6 compression strength. Engineering stress was
determined by dividing the force measured by nanoindentation
tester over pillars cross-section area. Hence, we have measured the
pillars diameters before and after the compression test with the use
of SEM. Furthermore, in order to determine engineering strain, the
ratio of displacement obtained from nanoindentation to the initial
height of a pillar measured by atomic force microscope (AFM) was
obtained.
3.6. Tribological studies Tribological tests in paper I and paper II were carried out using
universal tribometer (Rtec Instruments, San Jose, USA) equipped
with a normal load (Fn) sensor in the range of 1-100 N and
frictional force (Fx) sensor in the range of 1-50 N.
High temperature tribology
For paper 1, high temperature furnace with asbestos lining
was used to perform high temperature tribological tests. A ball-on-
disc sliding tests were performed on CuMoTaWV RHEA with
dimensions of 12 mm diameter and 5 mm thickness. The
tribological tests were performed at room temperature (RT), 200 oC, 400 oC and 600 oC for sliding distance of 200 m. The
tribological tests were performed against Si3N4 counter-ball with a
diameter of 9.5 mm. The counter-ball was washed in ethanol and
distilled water in ultrasonic machine followed by drying in oven at
70 oC. A normal load of 5 N was applied in all test. The counter-
ball and specimen was brought in contact after the desired test
temperature was reached. For repeatability of the tribotests, three
tests/condition were conducted and the average COF and wear rate
-
34
were reported. The average depth of the resulting wear track was
measured using optical profilometry (Wyko) to calculate the
specific wear rate at each temperature. At least 8 wear track depth
was measured for each tribotest. The wear rate, expressed as
mm3/Nm, was measured by dividing wear volume with normal
load and sliding distance.
Room temperature tribotest
For paper II, RT tribotest of CuMoTaWV RHEF was
performed in the ball-on-disc setup at 1 N normal load and a sliding
speed of 0.1 m/s. A counter-ball of E52100 alloy steel (Grade 25,
700-880 HV) with a diameter of 6.3 mm was used after cleaning
with ethanol in a ultrasonicator machine for 10 mins followed by
drying in an oven at 80 oC for 30 mins. The average depth of wear
was measured using optical profilometry (Wyko) to calculate the
wear rate expressed in mm3/Nm.
3.7. Scanning electron microscopy (SEM) SEM analysis was performed using JSM-IT300 (JEOL,
Tokyo, Japan) and Magellan 400 XHR-SEM (FEI Company,
Eindhoven, Netherlands) based on the requirement of
magnification. In paper I, JSM-IT300 was used to characterize the
microstructure of CuMoTaWV RHEA and the resulting wear
tracks after high temperature tribotests. The accelerating voltage
for microstructural analysis and wear track analysis from tribotest
was set to 15 kV and 10 kV, respectively.
In paper II, Magellon 400 was used to characterize the coating
surface and cross-section morphology of CuMoTaWV RHEF; and
JSM-IT300 was used to characterize the wear tracks resulting from
tribotests.
3.8. XPS measurements X-ray photoelectron spectra (XPS) were recorded using a
PerkinElmer PHI 5600 ci spectrometer with a standard Al−Kα
source (1486.6 eV) working at 250 W. The working pressure was
-
35
set to 5 × 10−8 Pa. The spectrometer was calibrated by assuming
the binding energy (BE) of the Au 4f7/2 line to be 84.0 eV with
respect to the Fermi level. Extended spectra (survey) were
collected in the range 0−1300 eV (187.85 eV pass energy, 0.5 eV
step, 0.025 s·step−1). Detailed spectra were recorded for the
following regions: V 2p, Mo 3d, Ta 4f, W 4f, Cu 2p and O1s (23.5
eV pass energy, 0.1 eV step, 0.2 s·step−1). The atomic percentage,
after a Shirley type background subtraction, was evaluated by
using the PHI sensitivity factors. The sample was analyzed before
and after 2 min Ar+ sputtering at 3.5 keV with an argon partial
pressure of 5 x 10-8 mbar and a rastered area of 2.5 x 2.5 mm.
3.9. Rutherford backscattering spectrometry In paper II, Rutherford Backscattering Spectrometry (RBS)
with a 1.8 or 2.0 MeV 4He+ beam in IBM geometry was used to
measure the composition homogeneity in the CuMoTaWV RHEF.
3.10. Atomic force microscopy (AFM) In paper II, the AFM analysis was used to study the surface
morphology and surface roughness of CuMoTaWV RHEF. The
AFM measurements were performed using ambient conditions
with an NTEGRA AFM (NT-MDT) in semi-contact mode using a
polysilicon lever, monocrystal silicon probe (HA_NC series) with
a tip height of 10 μm, nominal tip radius less than 10 nm, and a
measured resonance frequency of 240.3 Hz.
3.11. DFT calculations In paper II, density-functional theory (DFT) was applied to
predict the lattice structure and mechanical properties of
CuMoTaWV RHEF. Based on experimental XRD analysis, DFT
calculation was used to compare the theoretical and experimental
peak positions (2θ) of different planes.
-
36
4. Summary of appended papers
Paper I Title: High temperature tribology of CuMoTaWV high entropy
alloy.
In this work, an equiatomic CuMoTaWV high entropy alloy was
developed using spark plasma sintering (SPS) of elemental powder
mixture at 1400 oC. The objective of this paper was to see how the
microstructural changes at different temperature effects the
tribological properties. The resulting sintered alloy showed
formation of BCC solid solution along with V-rich phases with an
average hardness of 600 HV and 900 HV, respectively. Room
temperature (RT) ball-on-disc sliding wear against alloy steel at 5
N load showed negligible wear rate. Thus, Si3N4 as a counter-ball
was used against sintered CuMoTaWV high entropy alloy in
sliding motion to induce more wear. High temperature tribological
studies of the sintered alloy showed wear properties specific to test
temperatures. The tribological tests from RT to 600 oC showed an
increasing average COF from RT to 400 oC and then decreased to
at 600 oC. The wear rate was found to be lower at RT and 400 oC,
and slightly higher at 200 oC and 600 oC. The wear surfaces were
analyzed to elucidate the wear mechanism at each test temperature.
The wear of CuMoTaWV alloy was governed by adhesive wear at
RT, adhesive with mild abrasion wear at 200 oC, and oxidative
wear at 400 oC and 600 oC. The CuMoTaWV alloy showed
adaptive wear behavior at test temperatures of 400 oC and 600 oC.
At 400 oC, the adaptive behavior was found to be due to the
formation of CuO, resulting in reducing the wear; while at 600 oC,
the transformation of V-rich zones to elongated magneli phases of
V2O5 reduced the COF to 0.54.
Author’s Contribution:
The author planned and performed all the experimental work. The
manuscript was written together with the corresponding author.
-
37
Paper II Title: Synthesis and mechanical characterisation of CuMoTaWV
refractory high entropy film by magnetron sputtering.
This work involves the development of CuMoTaWV refractory
high entropy film (RHEF) using DC-magnetron sputtering. The
objective of this work was to see the effect of a single-alloy target
on the nanocrystallinity and mechanical properties of thin films.
Alloy target of equiatomic CuMoTaWV was partially sintering
using SPS process. The developed target was deposited on two
different substrates of silicon and 304 stainless steel. The deposited
CuMoTaWV RHEF showed formation of BCC solid solution with
a lattice parameter of 3.18 Å, which was found to be in good
agreement with the one for the DFT optimized SQS (3.16 Å). The
film’s compositional homogeneity was characterized using
Rutherford backscattering (RBS) and showed the presence of Cu
and V rich at the coating-substrate interface, while Mo, Ta and W
were found to be rich towards the surface of the film. Similarly,
XPS analysis of the film after 3 and 4 minutes of etching also
showed a high concentration of Mo, Ta and W situated towards the
top-surface of the film. The nanohardness measurements were
performed using nanoindentation at 10 mN load and showed an
average hardness and Young’s modulus 19.5 ± 2.3 GPa and 259.3
± 19.2 GPa, respectively. Nano-pillars from CuMoTaWV RHEF
with a diameter of 440 nm were developed using focused-ion beam
(FIB) analysis for compressional studies. The nano-pillar
compression analysis gave an average compressional strength and
Young’s modulus of 10.7 ± 0.8 GPa and 196 ± 10 GPa, respectively.
Such high hardness and compressional strength are due to the
nanocrystallinity and grain boundary strengthening of the films
with an average grain size of 18 nm. Tribological performance of
CuMoTaWV RHEF deposited on 304 stainless steel was studied
against alloy steel at RT. The tribotest showed removal of the film
after 15 m of sliding distance. Thus, the film was annealed at 300 oC to increase the adhesion of film with the substrate. After
-
38
annealing, the film showed a steady state COF of 0.25 and a low
wear rate of 6.4 x 10-6 mm3/Nm over a sliding distance of 50 m.
Author’s Contribution:
The author performed target preparation, coating synthesis,
nanoindentation measurements and tribological studies, analyzed all the
experimental data. The manuscript was written together with the
corresponding author.
-
39
5. Conclusions What are the microstructural characteristics of RHEA after SPS
and its effect on the tribological properties? Secondly, how does
the selection of different elements effects the properties at high
temperature?
The synthesized CuMoTaWV RHEA using SPS showed
composite structure with the formation of BCC HEA phase and the
presence of hard V-rich phase. The composite structure was
obtained due to consolidation of the elemental powder mixture at
low temperature of 1400 oC, which is much lower than the melting
point of each refectory element used. The tribological properties
showed a low frictional and wear properties at RT due to the
presence of hard V-rich phases and formation of tribofilm rich in
oxides of W and Ta. Adaptive wear behavior was observed during
tribology test at 400 oC and 600 oC. The oxidation of Cu at 400 oC
resulted in formation of CuO, which helped in reducing the wear;
and the hard V-rich zones transformed to elongated V2O5 phase at
600 oC, which helped in reducing the friction coefficient. This
study helped us prove that the microstructure and selection of
elements for RHEA is critical for tribological applications at
different temperatures.
How does the use of single partially sintered target effects the
nanocrystallinity and mechanical properties, such as hardness,
tribology and nano-pillar compression in RHEF?
CuMoTaWV RHEF was synthesized with DC-magnetron
sputtering using single partially sintered target. The resulting film
showed nanocrystalline BCC crystal structure with lattice
parameter and grain size of 3.18 Å and 18 nm, respectively. The
nanocrystallinity remained stable even after annealing at 300 oC.
The RBS and XPS studies showed that the films have high
concentration of Cu and V at the coating-substrate interface while
high concentration of Ta, W and Mo was found towards coating
-
40
surface. The compositional in-homogeneity can be linked to the
higher sputtering of Cu than refractory elements resulting in its
lower deposition rate towards the surface of the film. The
mechanical properties of RHEF showed an average hardness and
nano-pillar compressional strength of 19 ± 2.3 GPa and 10.7 ± 0.8
GPa, respectively. The high hardness and compressional strength
of RHEF has been attributed to the combined effect of
nanocrystallinity, solid solution strengthening and grain boundary
strengthening. The mechanical properties were found to be
superior to previously reported RHEF, which used elemental co-
sputtering of targets. The RT tribology studies showed low
frictional behavior and good adhesion of films after annealing at
300 oC.
-
41
6. Future work
For future work, the objective of doctorate thesis would be
twofold:
I. New compositions of RHEA would be synthesized
using SPS for high temperature mechanical and
tribological applications. Major emphasis of alloy
development would be to increase the number of
principle elements (up to eight refractory elements) to
see if the increase in configurational entropy can
stabilize the resulting solid solution and have a single-
phase crystal structure. Secondly, alloy design strategy
would involve developing light-weight RHEAs with
superior mechanical properties at wide-temperature
ranges.
II. The second part of the doctorate thesis would include
developing targets of developed new alloy
compositions and synthesizing RHEFs and its nitride
films, and studying its nano-mechanical and high
temperature tribological properties. RHEFs and its
nitrides tend to have high frictional coefficient. New
synthesis strategy would include co-sputtering multi-
principal element target with other targets, such as
MoS2 or graphite, to increasing the lubricating
properties of RHEFs.
-
42
-
43
7. References
[1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H.
Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple
principal elements: Novel alloy design concepts and outcomes, Adv.
Eng. Mater. 6 (2004) 299–303+274. doi:10.1002/adem.200300567.
[2] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural
development in equiatomic multicomponent alloys, Mater. Sci. Eng.
A. 375–377 (2004) 213–218. doi:10.1016/j.msea.2003.10.257.
[3] S. Ranganathan, Alloyed pleasures : Multimetallic cocktails, Curr. Sci.
85 (2003) 1404–1406.
[4] R.A. Swalin, Thermodynamics of Solids, 2nd Edn., Wiley, NY, 1991.
[5] D.B. Miracle, J.D. Miller, O.N. Senkov, C. Woodward, M.D. Uchic, J.
Tiley, Exploration and development of high entropy alloys for
structural applications, Entropy. 16 (2014) 494–525.
doi:10.3390/e16010494.
[6] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys
and related concepts, Acta Mater. 122 (2017) 448–511.
doi:10.1016/j.actamat.2016.08.081.
[7] D.A. Porter, K.. Easterling, S. M.Y, Phase transformation in metals
and alloys, 3rd Editio, CRC Press, 2009.
[8] J. Yeh, Recent progress in high-entropy alloys, Ann. Chim. – Sci. Des
Mater. 31 (2006) 633–648. doi:10.3166/acsm.31.633-648.
[9] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H.
Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple
principal elements: Novel alloy design concepts and outcomes, Adv.
Eng. Mater. 6 (2004) 299–303+274. doi:10.1002/adem.200300567.
[10] B.Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid-Solution
Phase Formation Rules for Multi-component Alloys **, Adv. Eng.
Mater. 10 (2008) 534–538. doi:10.1002/adem.200700240.
[11] Z. Yong, Z. Yunjun, H.U.I. Xidong, W. Meiling, C. Guoliang, Minor
alloying behavior in bulk metallic glasses and high-entropy alloys, Sci.
China Ser. G Physics, Mech. Astron. 51 (2008) 427–437.
doi:10.1007/s11433-008-0050-5.
[12] J. Yeh, S. Chen, J. Gan, S. Lin, T. Chin, Formation of Simple Crystal
Structures in Cu-Co-Ni-Cr-Al-Fe-Ti-V Alloys with Multiprincipal
Metallic Elements, Metall. Mater. Trans. A. 35 (2004) 2533–2536.
-
44
[13] J. Yeh, S. Chang, Y. Hong, S. Chen, S. Lin, Anomalous decrease in X-
ray diffraction intensities of Cu–Ni–Al–Co–Cr–Fe–Si alloy systems
with multi-principal elements, Mater. Chem. Phys. 103 (2007) 41–46.
doi:10.1016/j.matchemphys.2007.01.003.
[14] Y.J. Zhou, Y. Zhang, Y.L. Wang, G.L. Chen, Solid solution alloys of
AlCoCrFeNiTix with excellent room-temperature mechanical
properties, Appl. Phys. Lett. 90 (2007) 181904.
doi:10.1063/1.2734517.
[15] J.W. Yeh, Y.L. Chen, S.J. Lin, S.K. Chen, High-Entropy Alloys – A
New Era of Exploitation, Mater. Sci. Forum. 560 (2007) 1–9.
doi:10.4028/www.scientific.net/MSF.560.1.
[16] P. Huang, J. Yeh, T.-T. Shun, S.-K. Chen, Multi-Principal-Element
Alloys with Improved Oxidation and Wear Resistance for Thermal
Spray Coating, Adv. Eng. Mater. 6 (2004) 74–78.
doi:10.1002/adem.200300507.
[17] C. Hsu, J. Yeh, S. Chen, T. Shun, Wear Resistance and High-
Temperature Compression Strength of Fcc CuCoNiCrAl 0 . 5 Fe Alloy
with Boron Addition, Metall. Mater. Trans. A. 35A (2004) 1465–1469.
[18] C. Tong, M. Chen, S. Chen, J. Yeh, T. Shun, S. Lin, S. Chang,
Mechanical Performance of the Al x CoCrCuFeNi High-Entropy Alloy
System with Multiprincipal Elements, 36 (2005) 1263–1271.
[19] O.N. Senkov, G.B. Wilks, D.B. Miracle, C.P. Chuang, P.K. Liaw,
Refractory high-entropy alloys, Intermetallics. 18 (2010) 1758–1765.
doi:10.1016/j.intermet.2010.05.014.
[20] J.P. Couzinié, G. Dirras, L. Perrière, T. Chauveau, E. Leroy, Y.
Champion, I. Guillot, Microstructure of a near-equimolar refractory
high-entropy alloy, Mater. Lett. 126 (2014) 285–287.
doi:10.1016/j.matlet.2014.04.062.
[21] Y.Y. Chen, T. Duval, U.D. Hung, J.W. Yeh, H.C. Shih, Microstructure
and electrochemical properties of high entropy alloys—a comparison
with type-304 stainless steel, Corros. Sci. 47 (2005) 2257–2279.
doi:https://doi.org/10.1016/j.corsci.2004.11.008.
[22] S. Varalakshmi, M. Kamaraj, B.S. Murty, Synthesis and
characterization of nanocrystalline AlFeTiCrZnCu high entropy solid
solution by mechanical alloying, J. Alloys Compd. 460 (2008) 253–
257. doi:10.1016/j.jallcom.2007.05.104.
[23] B.S. Murty, J.W. Yeh, S. Ranganathan, Chapter 5 - Synthesis and
Processing, in: B.S. Murty, J.W. Yeh, S. Ranganathan (Eds.), High
Entropy Alloy., Butterworth-Heinemann, Boston, 2014: pp. 77–89.
-
45
doi:https://doi.org/10.1016/B978-0-12-800251-3.00005-5.
[24] S. Mohanty, N.P. Gurao, K. Biswas, Sinter ageing of equiatomic
Al20Co20Cu20Zn20Ni20 high entropy alloy via mechanical alloying,
Mater. Sci. Eng. A. 617 (2014) 211–218.
doi:10.1016/j.msea.2014.08.046.
[25] B. Gwalani, R.M. Pohan, J. Lee, B. Lee, R. Banerjee, H.J. Ryu, High-
entropy alloy strengthened by in situ formation of entropy- stabilized
nano-dispersoids, Sci. Rep. (2018) 1–9. doi:10.1038/s41598-018-
32552-6.
[26] B. Kang, J. Lee, H. Jin, S. Hyung, Materials Science & Engineering A
Ultra-high strength WNbMoTaV high-entropy alloys with fi ne grain
structure fabricated by powder metallurgical process, Mater. Sci. Eng.
A. 712 (2018) 616–624. doi:10.1016/j.msea.2017.12.021.
[27] A. Zhang, J. Han, B. Su, J. Meng, A novel CoCrFeNi high entropy
alloy matrix self-lubricating composite, J. Alloys Compd. 725 (2017)
700–710. doi:10.1016/j.jallcom.2017.07.197.
[28] S. Cao, J. Zhou, L. Wang, Y. Yu, B. Xin, Microstructure, mechanical
and tribological property of multi-components synergistic self-
lubricating NiCoCrAl matrix composite, Tribol. Int. (2018).
doi:10.1016/j.triboint.2018.10.050.
[29] D. Depla, S. Mahieu, Reactive Sputter Deposition, Springer Berlin
Heidelberg, 2008. doi:10.1007/978-3-540-76664-3.
[30] X.H. Yan, J.S. Li, W.R. Zhang, Y. Zhang, A brief review of high-
entropy films, Mater. Chem. Phys. 210 (2018) 12–19.
doi:10.1016/j.matchemphys.2017.07.078.
[31] W. Li, P. Liu, P.K. Liaw, Microstructures and properties of high-
entropy alloy films and coatings: A review, Mater. Res. Lett. 6 (2018)
199–229. doi:10.1080/21663831.2018.1434248.
[32] A.D. Pogrebnjak, V.M. Beresnev, K. V. Smyrnova, Y.O. Kravchenko,
P. V. Zukowski, G.G. Bondarenko, The influence of nitrogen pressure
on the fabrication of the two-phase superhard nanocomposite
(TiZrNbAlYCr)N coatings, Mater. Lett. 211 (2018) 316–318.
doi:10.1016/j.matlet.2017.09.121.
[33] S.Y. Lin, S.Y. Chang, Y.C. Huang, F.S. Shieu, J.W. Yeh, Mechanical
performance and nanoindenting deformation of (AlCrTaTiZr)NC y
multi-component coatings co-sputtered with bias, Surf. Coatings
Technol. 206 (2012) 5096–5102. doi:10.1016/j.surfcoat.2012.06.035.
[34] M.I. Lin, M.H. Tsai, W.J. Shen, J.W. Yeh, Evolution of structure and
-
46
properties of multi-component (AlCrTaTiZr)Ox films, Thin Solid
Films. 518 (2010) 2732–2737. doi:10.1016/j.tsf.2009.10.142.
[35] M. Braic, V. Braic, M. Balaceanu, C.N. Zoita, A. Vladescu, E.
Grigore, Characteristics of (TiAlCrNbY)C films deposited by reactive
magnetron sputtering, Surf. Coatings Technol. 204 (2010) 2010–2014.
doi:10.1016/j.surfcoat.2009.10.049.
[36] Y.S. Jhong, C.W. Huang, S.J. Lin, Effects of CH4flow ratio on the
structure and properties of reactively sputtered
(CrNbSiTiZr)Cxcoatings, Mater. Chem. Phys. 210 (2018) 348–352.
doi:10.1016/j.matchemphys.2017.08.002.
[37] H. Kim, S. Nam, A. Roh, M. Son, M. Ham, J. Kim, H. Choi,
Mechanical and electrical properties of NbMoTaW refractory high-
entropy alloy thin films, Int. J. Refract. Metals Hard Mater. 80 (2019)
286–291. doi:10.1016/j.ijrmhm.2019.02.005.
[38] S. Fritze, C.M. Koller, L. Von Fieandt, P. Malinovskis, K. Johansson,
E. Lewin, P.H. Mayrhofer, U. Jansson, Influence of Deposition
Temperature on the Phase Evolution of HfNbTiVZr High-Entropy
Thin Films, (2019) 10–17. doi:10.3390/ma12040587.
[39] H.-T. Hsueh, W.-J. Shen, M.-H. Tsai, J.-W. Yeh, Effect of nitrogen
content and substrate bias on mechanical and corrosion properties of
high-entropy films (AlCrSiTiZr)100−xNx, Surf. Coatings Technol.
206 (2012) 4106–4112. doi:10.1016/j.surfcoat.2012.03.096.
[40] H.W. Chang, P.K. Huang, J.W. Yeh, A. Davison, C.H. Tsau, C.C.
Yang, Influence of substrate bias, deposition temperature and post-
deposition annealing on the structure and properties of multi-principal-
component (AlCrMoSiTi)N coatings, Surf. Coatings Technol. 202
(2008) 3360–3366. doi:10.1016/j.surfcoat.2007.12.014.
[41] P.K. Huang, J.W. Yeh, Effects of substrate temperature and post-
annealing on microstructure and properties of (AlCrNbSiTiV)N
coatings, Thin Solid Films. 518 (2009) 180–184.
doi:10.1016/j.tsf.2009.06.020.
[42] A.D. Pogrebnjak, I. V. Yakushchenko, O. V. Bondar, O. V. Sobol’,
V.M. Beresnev, K. Oyoshi, H. Amekura, Y. Takeda, Influence of
implantation of Au− ions on the microstructure and mechanical
properties of the nanostructured multielement (TiZrHf VNbTa)N
coating, Phys. Solid State. 57 (2015) 1559–1564.
doi:10.1134/S1063783415080259.
[43] Z. Chang, Structure and properties of duodenary (
TiVCrZrNbMoHfTaWAlSi ) N coatings by reactive magnetron
sputtering, Mater. Chem. Phys. 220 (2018) 98–110.
-
47
doi:10.1016/j.matchemphys.2018.08.068.
[44] Y. Zou, H. Ma, R. Spolenak, Ultrastrong ductile and stable high-
entropy alloys at small scales, Nat. Commun. 6 (2015) 1–8.
doi:10.1038/ncomms8748.
[45] Y. Zou, J.M. Wheeler, H. Ma, P. Okle, R. Spolenak, Nanocrystalline
High-Entropy Alloys: A New Paradigm in High-Temperature Strength
and Stability, Nano Lett. 17 (2017) 1569–1574.
doi:10.1021/acs.nanolett.6b04716.
[46] S. Mridha, M. Komarasamy, S. Bhowmick, R.S. Mishra, S.
Mukherjee, Small-Scale Plastic Deformation of Nanocrystalline High
Entropy Alloy, Entropy. 20 (2018) 1–6. doi:10.3390/e20110889.
[47] L. Gao, W. Liao, H. Zhang, J. Surjadi, D. Sun, Y. Lu, Microstructure,
Mechanical and Corrosion Behaviors of CoCrFeNiAl0.3 High Entropy
Alloy (HEA) Films, Coatings. 7 (2017) 156.
doi:10.3390/coatings7100156.
[48] K.H. Cheng, C.H. Lai, S.J. Lin, J.W. Yeh, Structural and mechanical
properties of multi-element (AlCrMoTaTiZr)Nxcoatings by reactive
magnetron sputtering, Thin Solid Films. 519 (2011) 3185–3190.
doi:10.1016/j.tsf.2010.11.034.
[49] V. Braic, A. Vladescu, M. Balaceanu, C.R. Luculescu, M. Braic,
Nanostructured multi-element (TiZrNbHfTa)N and (TiZrNbHfTa)C
hard coatings, Surf. Coatings Technol. 211 (2012) 117–121.
doi:10.1016/j.surfcoat.2011.09.033.
[50] W.-J. Shen, M.-H. Tsai, J.-W. Yeh, Machining Performance of
Sputter-Deposited (Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 High-
Entropy Nitride Coatings, Coatings. 5 (2015) 312–325.
doi:10.3390/coatings5030312.
[51] V. Bushlya, D. Johansson, F. Lenrick, J. Ståhl, F. Schultheiss, Wear
mechanisms of uncoated and coated cemented carbide tools in
machining lead-free silicon brass, Wear. 376–377 (2017) 143–151.
doi:10.1016/j.wear.2017.01.039.
[52] R. Yu, R. Huang, C. Lee, F. Shieu, Synthesis and characterization of
multi-element oxynitride semiconductor film prepared by reactive
sputtering deposition, Appl. Surf. Sci. 263 (2012) 58–61.
[53] R. Lin, T. Lee, D. Wu, Y. Lee, A Study of Thin Film Resistors
Prepared Using Ni-Cr-Si-Al-Ta High Entropy Alloy, Adv. Mater. Sci.
Eng. Article ID (2015) 7 pages.
[54] P.-C. Lin, C.-Y. Cheng, J.-W. Yeh, T.-S. Chin, Soft Magnetic
-
48
Properties of High-Entropy Fe-Co-Ni-Cr-Al-Si Thin Films, Entropy.
18 (2016). doi:10.3390/e18080308.
[55] P.K. Huang, J.W. Yeh, Inhibition of grain coarsening up to 1000 C in
(AlCrNbSiTiV)N superhard coatings, Scr. Mater. 62 (2010) 105–108.
doi:10.1016/j.scriptamat.2009.09.015.
[56] W.J. Shen, M.H. Tsai, K.Y. Tsai, C.C. Juan, C.W. Tsai, J.W. Yeh,
Y.S. Chang, Superior Oxidation Resistance of
(Al0.34Cr0.22Nb0.11Si0.11Ti0.22)50N50 High-Entropy Nitride, J.
Electrochem. Soc. 160 (2013) C531–C535. doi:10.1149/2.028311jes.
[57] Y.F. Ye, Q. Wang, J. Lu, C.T. Liu, Y. Yang, High-entropy alloy:
challenges and prospects, Mater. Today. 19 (2016) 349–362.
doi:10.1016/j.mattod.2015.11.026.
[58] O.N. Senkov, D.B. Miracle, K.J. Chaput, J.P. Couzinie, Development
and exploration of refractory high entropy alloys - A review, J. Mater.
Res. 33 (2018) 3092–3128. doi:10.1557/jmr.2018.153.
[59] O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical
properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20
refractory high entropy alloy, Intermetallics. 19 (2011) 698–706.
doi:10.1016/j.intermet.2011.01.004.
[60] O.N. Senkov, S. V. Senkova, C. Woodward, D.B. Miracle, Low-
density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-
Zr system: Microstructure and phase analysis, Acta Mater. 61 (2013)
1545–1557. doi:10.1016/j.actamat.2012.11.032.
[61] E.M. Savitskii, G.S. Burkhanov, Physical Metallurgy of Refractory
Metals and Alloys, Springer, Boston, MA, 1995. doi:10.1007/978-1-
4684-1572-8.
[62] W. Guo, B. Liu, Y. Liu, T. Li, A. Fu, Q. Fang, Y. Nie, Microstructures
and mechanical properties of ductile NbTaTiV refractory high entropy
alloy prepared by powder metallurgy, J. Alloys Compd. 776 (2019)
428–436. doi:10.1016/j.jallcom.2018.10.230.
[63] T.K. Tsao, A.C. Yeh, C.M. Kuo, K. Kakehi, H. Murakami, J.W. Yeh,
S.R. Jian, The High Temperature Tensile and Creep Behaviors of High
Entropy Superalloy, Sci. Rep. 7 (2017) 1–9. doi:10.1038/s41598-017-
13026-7.
[64] C.C. Juan, M.H. Tsai, C.W. Tsai, C.M. Lin, W.R. Wang, C.C. Yang,
S.K. Chen, S.J. Lin, J.W. Yeh, Enhanced mechanical properties of
HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys,
Intermetallics. 62 (2015) 76–83. doi:10.1016/j.intermet.2015.03.013.
-
49
[65] S. Sheikh, S. Shafeie, Q. Hu, J. Ahlström, C. Persson, J. Veselý, J.
Zýka, U. Klement, S. Guo, Alloy design for intrinsically ductile
refractory high-entropy alloys, J. Appl. Phys. 120 (2016).
doi:10.1063/1.4966659.
[66] N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo,
H.Z. Fu, Microstructure and mechanical properties of refractory high
entropy (Mo0.5NbHf0.5ZrTi) BCC/M5Si3 in-situ compound, J.
Alloys Compd. 660 (2016) 197–203.
doi:10.1016/j.jallcom.2015.11.091.
[67] N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo,
H.Z. Fu, Microstructure and mechanical properties of in-situ MC-
carbide particulates-reinforced refractory high-entropy
Mo0.5NbHf0.5ZrTi matrix alloy composite, Intermetallics. 69 (2016)
74–77. doi:10.1016/j.intermet.2015.09.011.
[68] Y. Zhang, Y. Liu, Y. Li, X. Chen, H. Zhang, Microstructure and
mechanical properties of a refractory HfNbTiVSi0.5 high-entropy
alloy composite, Mater. Lett. 174 (2016) 82–85.
doi:10.1016/j.matlet.2016.03.092.
[69] Y. Liu, Y. Zhang, H. Zhang, N. Wang, X. Chen, H. Zhang, Y. Li,
Microstructure and mechanical properties of refractory
HfMo0.5NbTiV0.5Six high-entropy composites, J. Alloys Compd.
694 (2017) 869–876. doi:10.1016/j.jallcom.2016.10.014.
[70] N.Y. Yurchenko, N.D. Stepanov, S. V Zherebtsov, M.A. Tikhonovsky,
G.A. Salishchev, Structure and mechanical properties of B2 ordered
refractory AlNbTiVZrx (x=0–1.5) high-entropy alloys, Mater. Sci.
Eng. A. 704 (2017) 82–90. doi:10.1016/j.msea.2017.08.019.
[71] N.D. Stepanov, N.Y. Yurchenko, D.G. Shaysultanov, G.A. Salishchev,
M.A. Tikhonovsky, Effect of Al on structure and mechanical
properties of AlxNbTiVZr (x=0, 0.5, 1, 1.5) entropy alloys, Mater. Sci.
Technol. 31 (2015) 1184–1193.
doi:10.1179/1743284715Y.0000000032.
[72] O.N. Senkov, S. V Senkova, D.B. Miracle, C. Woodward, Mechanical
properties of low-density , refractory multi-principal element alloys of
the Cr–Nb–Ti–V–Zr system, Mater. Sci. Eng. A. 565 (2013) 51–62.
doi:10.1016/j.msea.2012.12.018.
[73] L. Lilensten, J. Couzinié, J. Bourgon, L. Perrière, G. Dirras, F. Prima,
I. Guillot, Design and tensile properties of a bcc Ti-rich high-entropy
alloy with transformation-induced plasticity, Mater. Res. Lett. 5 (2017)
110–116. doi:10.1080/21663831.2016.1221861.
[74] H. Huang, Y. Wu, J. He, H. Wang, X. Liu, K. An, W. Wu, Z. Lu,
-
50
Phase-Transformation Ductilization of Brittle High-Entropy Alloys via
Metastability Engineering, Adv. Mater. 29 (2017) 1–7.
doi:10.1002/adma.201701678.
[75] A. Poulia, E. Georgatis, A. Lekatou, A.E. Karantzalis, Microstructure
and wear behavior of a refractory high entropy alloy, Int. J. Refract.
Met. Hard Mater. 57 (2016) 50–63. doi:10.1016/j.ijrmhm.2016.02.006.
[76] C. Mathiou, A. Poulia, E. Georgatis, A.E. Karantzalis, Microstructural
features and dry - Sliding wear response of MoTaNbZrTi high entropy
alloy, Mater. Chem. Phys. 210 (2018) 126–135.
doi:10.1016/j.matchemphys.2017.08.036.
[77] V. Braic, M. Balaceanu, M. Braic, A. Vladescu, S. Panseri, A. Russo,
Characterization of multi-principal-element (TiZrNbHfTa)N and
(TiZrNbHfTa)C coatings for biomedical applications, J. Mech. Behav.
Biomed. Mater. 10 (2012) 197–205.
doi:10.1016/j.jmbbm.2012.02.020.
[78] A.D. Pogrebnjak, I. V. Yakushchenko, A.A. Bagdasaryan, O. V.
Bondar, R. Krause-Rehberg, G. Abadias, P. Chartier, K. Oyoshi, Y.
Takeda, V.M. Beresnev, O. V. Sobol, Microstructure, physical and
chemical properties of nanostructured (Ti-Hf-Zr-V-Nb)N coatings
under different deposition conditions, Mater. Chem. Phys. 147 (2014)
1079–1091. doi:10.1016/j.matchemphys.2014.06.062.
[79] X.B. Feng, J.Y. Zhang, Y.Q. Wang, Z.Q. Hou, K. Wu, G. Liu, J. Sun,
Size effects on the mechanical properties of nanocrystalline
NbMoTaW refractory high entropy alloy thin films, Int. J. Plast. 95
(2017) 264–277. doi:10.1016/j.ijplas.2017.04.013.
[80] X. Feng, J. Zhang, Z. Xia, W. Fu, K. Wu, G. Liu, J. Sun, Stable
nanocrystalline NbMoTaW high entropy alloy thin films with
excellent mechanical and electrical properties, Mater. Lett. 210 (2018)
84–87. doi:10.1016/j.matlet.2017.08.129.
[81] N. Tüten, D. Canadinc, A. Motallebzadeh, B. Bal, Intermetallics
Microstructure and tribological properties of TiTaHfNbZr high
entropy alloy coatings deposited on Ti e 6Al e 4V substrates,
Intermetallics. 105 (2019) 99–106.
doi:10.1016/j.intermet.2018.11.015.
[82] Y. Zou, Nanomechanical studies of high-entropy alloys, J. Mater. Res.
33 (2018) 3035–3054. doi:10.1557/jmr.2018.155.
-
Synthesis and Characterisation of High Entropy Alloy and Coating
Sajid Ali Alvi
Engineering Materials
Department of Engineering Sciences and MathematicsDivision of Materials Science
ISSN 1402-1757ISBN 978-91-7790-394-9 (print)ISBN 978-91-7790-395-6 (pdf)
Luleå University of Technology 2019
LICENTIATE T H E S I S
Sajid Ali A
lvi Synthesis and Characterisation of H
igh Entropy A
lloy and Coating